This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Human herpesvirus-8 (HHV-8), also known as Karposi's sarcoma-associated human herpesvirus (KSHV), is a gammaherpesvirus that is the causative agent of various lymphoproliferative disorders including Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. KSHV encodes the viral processivity factor, PF-8, which binds to DNA and is essential for maintaining contact between the viral polymerase Pol-8 and DNA during DNA synthesis. The crystal structures of a processivity factor from the alpha herpesvirus, HSV-1, and the beta herpesvirus, HCMV, revealed structures similar to that of the eukaryotic processivity factor, proliferating cell nuclear antigen (PCNA), except in their quaternary rearrangement. PCNA forms a trimeric ring around DNA while the processivity factors from HSV-1 and HCMV are a monomer and dimer respectively and do not require a clamp loader or ATP in order to be loaded onto DNA. We have determined the crystal structure of PF-8 to 2.8[unreadable], and show that PF-8 forms a head-to-head homodimer that appears to fall, functionally, somewhere between the structures of UL42 and UL44. Based on the structural data, we are able to identify possible sites for interactions with DNA and polymerase, to propose a model for the simultaneous interaction of all three components of the complex. In addition, the availability of crystal structures for all three herpesvirus classes provides new insights into comparative structure and function.